1. Field of the Invention
The present invention relates generally to selective and extended photothermolysis for cosmetic, health and dermatology conditions, and more particularly, to a portable device for photo-inducing damage to cellular structures for hair removal.
2. Brief Description of the Background Art
Electromagnetic energy, particularly in the optical band of 400 nm to 1200 nm, has been used for treatment of many skin related diseases as well as for cosmetic procedures, such as, hair removal, spider veins, tattoos, port wine stains, skin rejuvenation and photodynamic therapy. Laser and light-based removal of hair, both in men and women, is widely accepted as a successful approach. In today's market place, manufacturers have focused on four laser-based systems: 1) alexandrite (755 nm); 2) neodymium-doped yttrium aluminum garnet (1064 nm); 3) laser diodes (810 nm); and 4) a broad band Intense Pulsed Light (IPL) source. Generally, these systems provide reduction in the growth cycle of removed hair. Multiple treatments have been found to improve upon longevity of the hair free period. An endpoint for an acceptable treatment requires destruction of pluripotential follicular stem cells and not merely evaporation of the hair shaft.
Recent data suggests that the stem cells are found in upper bulb and bulge regions of the hair follicle. Indeed there may be other areas not yet identified. Laser Hair Removal (LHR) procedures must target these regions of the stem cells, as they are responsible for hair growth. Several techniques have been developed for destruction of stem cells.
Laser ablation, not typically used for photoepilation, uses high energy short pulses to raise the temperature of the stem cell above that required for evaporation, however, the target and the absorber must be collocated. Selective Photothermolysis (SP) exploits dissimilar absorption coefficients of the photo absorbers and surrounding tissue. However, use of SP for destroying the stem cells responsible for hair growth is compounded because the photo-absorbing chromophore, melanin, is found both in the follicular stem cells and the epidermis. Melanin has a broad absorption spectrum and is responsible for pigmentation of the hair shaft and skin. SP techniques are effective if a concentration of melanin is higher, by a factor of five, in the target area. These techniques work particularly well for dark hair on light skin. However, unavoidable absorption of photons in the epidermis leads to heat, which needs to be removed to avoid damage to the epidermis. Consequently, innovative hand-pieces which chill the epidermis during treatment have been developed.
Destruction of cells through thermal denaturing requires that a target temperature exceeding 70° C. within the Thermal Relaxation Time (TRT) of the tissue. For the hair shaft, the TRT is in the range of 35 to 50 ms. Pulse widths exceeding the TRT permit diffusion of heat into surrounding tissue preventing the denaturing temperature from being reached due to heat leakage. Typically, LHR devices target about a ˜1 cm2 area of the skin, which is bombarded with photons. Some photons are absorbed in the epidermis, while the remaining migrate, via scattering, through the dermis and reach the melanin rich hair shaft and bulb region, where absorption leads to elevation of tissue temperature causing cell destruction. The photons scattered in the backward direction return back to the epidermis resulting in fluence levels exceeding the incident fluence.
Based on photon transport theory and clinical data, an optimum set of parameters can be established for a particular device. Unfortunately, these parameters are patient dependent and use of LHR devices remains an art.
A typical laser diode system will have a variable fluence between 20 to 60 J·cm−2, a pulse width in the range of 5 to 500 ms, and a treatment spot size of ˜1 cm2. The peak power of the source, which determines the size of the LHR system, is proportional to the product of fluence and spot area and inversely proportional to the pulse width. For example, a 100 μs pulse with a spot area of 1 cm2 requires a peak pulse power of 20 kW for a fluence of 20 J·cm2. Consequently, this leads to bulky and expensive machines, which need full medical facilities for operation. While the large diameter reduces treatment time and increases penetration depth into the dermis, it lacks the capability to selectively remove hair from a given area, i.e. to reduce hair density.
Another approach for permanent hair removal is based on Extended Selective Photothermolysis (ESP). The target to be denatured can be separated from a photo-absorber, known as a heat source. A closer study of the underlying thermal diffusive processes has led to use of longer pulses to produce a hot spot in the melanin rich hair shaft. The longer laser pulse produces a hot spot, which begins to heat the surrounding tissue, including the hair bulb and bulge. Pulse width is determined by the TRT and the Thermal Damage Time (TDT). Recent studies have indicated, particularly for techniques using the hair shaft for heat transmission, that a longer pulse width up to 1.5 s may be acceptable, which substantially decreases the peak power requirement. Several LHR systems with peak power up to 200 W using laser diode arrays are now on the market.
Other procedures for efficiently using the available photons in LHR devices include the use of highly reflective and thermally conductive applications to the skin prior to laser treatment. Ultrasonic massaging increases penetration of a dye into the epidermis. Pre-treatments can be used with any of the light-based techniques to enhance efficacy of hair removal, but adds extra time and cost to the treatment.
U.S. Pat. No. 7,118,563 to Weckwerth, the disclosure of which is incorporated herein by reference, discloses a rechargeable device suitable for providing therapeutic energy. However, the minimum spot size of 0.25 cm2 is too large for targeting single hair follicles and causes a reduction in the peak power requirement. The system disclosed by Weckwerth also lacks any imaging device for identifying a treatment area.
U.S. Pat. No. 7,220,254 to Altshchuler, the disclosure of which is incorporated herein by reference, teaches that existing technology can be packaged into a self-contained hand-held device for delivery of therapeutic energy to a skin treatment area and can be visualized by an image capturing system integrated into the hand-held device. The device combines discrete optical and electronic components to illuminate an area of the skin to facilitate imaging by a Charge Coupled Device/Complementary Metal Oxide Semiconductor (CCD/CMOS) device. Imaging and treatment optical paths are separated by a beam splitter. A more compact and user-friendly hand-held device, with few components, would be more desirable, particularly for the home market.
In fact, in keeping with this concept, U.S. Pub. No. 2007/0198004 Altschuler et al., the disclosure of which is incorporated herein by reference, addresses some of the above problems in disclosing a tethered hand-piece which may be more appropriate for the home market. However, conventional photo cosmetic devices do not include imaging capability and use lower power EMR sources having prolonged exposure times. For hair removal, such devices recommend power levels in the range of 20-500 W, which is not attainable by a single laser diode.